The IPAT equation, though phrased mathematically, is a simple conceptual expression of the factors that create environmental impact. IPAT is an accounting identity stating that environmental impact (I) is the product of three terms: 1) population (P); 2) affluence (A); and 3) technology (T). It is stated I = P x A x T or I=PAT. Generally credited to ecologist Paul Ehrlich, the IPAT formulation arose from a dispute in the early 1970s among the most prominent environmental thinkers of the day about the sources of environmental impact. Ehrlich and John Holdren identified population size and growth as the most urgent IPAT factor, whereas Barry Commoner argued that post-World War II production technologies were the dominant reason for environmental degradation.
Passing over more complex models, IPAT has been chosen by many scholars in both the social and natural sciences as a starting point for investigating interactions of population, economic growth, and technological change. Operationalizing IPAT terms has figured into work on consumption, agriculture, and energy decomposition analysis. IPAT variants have been used by the Intergovernmental Panel on Climate Change (IPCC) in energy-related carbon emission studies. IPAT variants since the 1990s have reversed the original 1970s view of the harmful role of technology. The World Resources Institute studies of industrial ecology reinterpreted IPAT to suggest that given increases in population and affluence, the T term of the IPAT equation then becomes an essential counterweight to P and A requiring environmentally effective technological choices to reduce environmental impact per unit of economic activity.
- Chertow, M. R., 2001. The IPAT Equation and Its Variants; Changing Views of Technology and Environmental Impact. Journal of Industrial Ecology, 4.4:13-29.
- Commoner, Barry, 1972. The Environmental Cost of Economic Growth. In: Population, Resources and the Environment. Government Printing Office, Washington, DC. pp. 339-63.
- Dietz, T. and E. A. Rosa, 1994. Rethinking the environmental impacts of population, affluence and technology. Human Ecology Review, 1(1).
- Ehrlich, Paul R. and John P. Holdren, 1971. Impact of Population Growth. Science, 171:1212-17.
- Fischer-Kowalski, M. and C. Amann, 2001. Beyond IPAT and Kuznets curves: Globalization as a vital factor in analysing the environmental impact of socio-economic metabolism. Population Environment, 23(1):7-47.
- IPCC, 2001. Special Report on Emissions Scenarios: a special report of Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.
- Waggoner, P. E. and J. H. Ausubel, 2002. A framework for sustainability science: a renovated IPAT identity. Proceedings of the National Academy of Science, 99(12):7860-5.
- York, R., E.A. Rosa and T. Dietz, 2003. STIRPAT, IPAT and ImPACT: Analytic tools for unpacking the driving forces of environmental impacts. Ecological Economics, 46(3):351-365.